Chinese police on Thursday night confirmed for the first time that three of the five Hong Kong booksellers who went missing last year are being held on the mainland for “illegal activities”.

A letter from an Interpol office in Guangdong province in southern mainland China said the three men being held "were suspected [of being] involved in a case relating to a person named Gui, and were involved in illegal activities on the mainland".

“Criminal compulsory measures were imposed on them and they were under investigation,” the statement said.

Lui Por, Cheung Jiping and Lam Wing-kei, who work for the Hong Kong publishing house Mighty Current, disappeared while in southern mainland China in October 2015.

Mighty Current and its subsidiary bookstore, Causeway Bay Books, specialise in gossipy titles that criticise mainland China’s Communist Party leaders.

Suspected abductions

It remains unclear how Lui, Cheung, Lam, and two more of their colleagues from the Hong Kong publishing company disappeared and ended up in mainland China.

The fourth bookseller, Gui Minhai, failed to return to Hong Kong from a holiday in Thailand in October.

Two weeks ago Gui appeared on Chinese state television, tearfully stating that he had voluntarily turned himself in to Chinese authorities over a fatal drink driving case from more than a decade ago.

The fifth bookseller, Lee Bo, went missing from Hong Kong in December. He resurfaced on the mainland almost immediately, calling his wife to say he was assisting in an investigation and would not be back anytime soon.

Thursday’s police statement contained an equally confusing letter from Lee, whom Hong Kong police have been trying to contact ever since his disappearance. The letter said that Lee “did not need to meet with them (the Hong Kong police) at the moment”.

‘Smoke and mirrors’

The international community has strongly condemned China’s detention of the booksellers.

On Friday, the European Parliament called for the “immediate safe release” of the five in a statement.

Amnesty International has accused Chinese authorities of using a “smoke and mirrors strategy” to hide the truth about the booksellers’ disappearance.

Britain handed Hong Kong back to China in 1997 under agreements that its broad freedoms, way of life and vaunted legal system would remain unchanged for 50 years. The “one country, two systems” formula accords Hong Kong a degree of autonomy and freedoms not enjoyed in mainland China, including freedom of speech.

However, in recent years have seen attempts by Beijing to undermine Hong Kong’s independence, including restrictions on its electoral process and a crackdown on pro-democracy protests in 2014.